1. Field of the Invention
The present invention relates to a method of fabricating a rigid flexible printed circuit board (PCB). More particularly, the present invention pertains to a method of fabricating a rigid flexible PCB, wherein a self-detachable adhesive tape used to separate a wafer from a substrate in the course of conventionally fabricating a semiconductor wafer, is employed to avoid inherent problems occurring in a conventional method of fabricating the rigid flexible PCB.
2. Description of the Prior Art
In accordance with the recent trend toward miniaturized, light, and slim electronic devices, light, slim, and high density PCBs are in demand. Accordingly, the use of a multilayered flexible PCB capable of satisfying the above demand is rapidly growing. However, a process of fabricating the multilayered flexible PCB is complicated and has many stages compared to a process of fabricating a general rigid PCB, thus costs and defective fractions are high. Of multilayered rigid PCBs, recently, a rigid flexible PCB having a flexible portion has been employed to produce folding-type mobile phones, thus it is expected that demand for rigid flexible PCBs will rapidly grow.
The flexible portion of the rigid flexible PCB reduces workability in a process of producing goods and increases the defective fractions. The development of special material and a process employing same contribute to solving the above problems, thereby assuring superiority over rival companies.
In order to fabricate the flexible PCB, the formation of microcircuits using thin and strong material must be feasible. In other words, the selection of material and process is important. Polyimide resin has mainly been used as the material because there is no material usable as a substitute for polyimide resin with regard to heat resistance, mechanical strength, fire retardancy, and electrical properties. However, currently, material capable of being used as a substitute for polyimide resin has been developed with regard to high frequency response, moisture resistance, dimensional stability, and cost.
With respect to technology of processing microcircuits of the flexible PCB, a pitch of a connection part of a flat panel display, such as an LCD, becomes fine so as to be changed from 33 μm to 25 μm in accordance with an increase in resolution, thus slimness of copper foils and base films has been achieved so that they are both 5 μm in thickness. According to data from JEITA (Japan Electronic and Information Technology Industries Association), it is predicted that the flexible PCB must have a minimum conductor width/minimum gap (L/S) of 15 μm/12 μm in the year 2006 and L/S of 10 μm/10 μm in the year 2008, and that a tape substrate must have L/S of 7 μm/7 μm in the year 2006 and L/S of 5 μm/5 μm in the year 2010.
To satisfy the above conditions, there remains a need to develop technologies for forming microcircuits or to develop novel processes or novel materials.
FIGS. 1a to 1i are sectional views illustrating a procedure of fabricating a rigid flexible PCB, according to conventional technology.
FIG. 1a is a sectional view of a flexible base substrate 101 on which circuit patterns 102 are formed.
In FIG. 1b, a coverlay 103 is partially or totally applied on the substrate 101.
In FIG. 1c, prepregs 104 or bonding sheets are layered on both sides of the substrate 101. Since the prepregs 104 or the bonding sheets are physically rigid, prepreg portions 105a, 105b of the resulting structure become rigid portions, and the remaining portion 106 of the resulting structure, on which the prepregs 104 are not layered, becomes a flexible portion 106.
In FIG. 1d, thin copper foils 107 are layered on both sides of the substrate 101. At this stage, since a height difference exists between the rigid portions 105a, 105b in which the prepregs 104 are layered and the flexible portion 106 in which the prepregs 104 are not layered, a portion of the copper foil layered on the flexible portion 106 sinks, causing a gap 108a between the copper foil 107 and the coverlay 103 on the flexible portion 106. On a portion of the substrate on which the coverlay 103 is not applied, a larger gap 108b is formed between the flexible substrate 101 and the copper foil 107.
In the layering of the copper foils 107, heat and pressure are simultaneously applied to the prepregs 104 after the copper foils 107 are layered so that the copper foils 107 are stuck to prepregs 104. At this time, the prepregs 104 are melted and thus flow from the rigid portions 105a, 105b to the flexible portion 106.
If the prepregs flow downward as described above, the prepreg material is present on the flexible portion, thus the minimum curvature radius of the flexible portion is not the same as the set design value.
In FIG. 1e, a via hole 109 is formed through a portion of the rigid portions 105a, 105b, and copper plating layers 112 are formed on a wall of the via hole 109 and on external layers of the substrate through a plating process. The formation of the via hole 109 is mostly conducted using laser drilling or mechanical drilling. Typically, after the via hole 109 is formed, a desmear process is implemented to remove impurities from around the via hole.
In FIG. 1f, a copper plating process is conducted on both sides of the substrate to form the copper plating layers 112 on surfaces of the substrate and on the wall of the via hole 109.
In FIG. 1g, dry films 111 are applied on both sides of the substrate. A portion of the copper foil 107 on the flexible portion 106 has already sunk, and portions of the dry films 111 on the sunk portions of the copper foils 107 sink, forming additional gaps 110a, 110b between the copper foils 107 and the dry films 111.
In FIG. 1h, the dry films 111 are exposed and developed to form etching resist patterns. At this stage, since the dry films 111 are not flat but have recesses, the dry films are nonuniformly exposed during an exposure process, resulting in the formation of undesirable etching resist patterns.
In FIG. 1i, etching is conducted to form circuit patterns on the copper foils 107, and etching resists are stripped, thereby creating the rigid flexible PCB.
In the fabrication of the rigid flexible PCB according to conventional technology, as shown in FIG. 1i, a height difference exists between the rigid portions 105a, 105b and the flexible portion 106, thus the prepregs flow down on the flexible portion 106 during the heating and pressurizing processes of FIG. 1d, causing gaps 108a, 108b, 110a, 110b between the copper foils 107 and the coverlay 103 and between the copper foils 107 and the dry films 111, resulting in defective products.
In order to avoid the problems of nonuniform exposure of the dry films 111 as shown in FIG. 1h, a laser direct imaging (LDI) process, in which exposure is conducted with a laser device used in conventional laser drilling, may be employed so as to expose recesses or corners, onto which light is not easily radiated, during the exposure. However, this process is problematic in that productivity is very poor.
With respect to this, Japanese Patent Laid-Open Publication No. 2000-332416 discloses a method of fabricating a PCB, in which an external base film is layered on a cable portion as well as on a rigid portion so as to protect the cable portion corresponding to a flexible portion in a rigid flexible multilayered PCB. The external base film is removed from the cable portion after various chemical treatments, making the requirement for a cover film, which protects the cable portion corresponding to the flexible portion from the chemical treatments conducted during the fabrication of the PCB, unnecessary.
Having been layered on only the rigid portion, the external base film is layered on the flexible portion as well as on the rigid portion in the above method, thereby avoiding problems caused by a height difference between the rigid portion and the flexible portion. In this regard, the external base film layered on the flexible portion must be removed afterwards, but the above patent does not suggest a method of removing the external base film. The external base film has adhesive strength due to heat and pressure in the course of fabricating the PCB, thus it is very difficult to remove the external base film from the cable portion corresponding to the flexible portion. Additionally, when it is removed, a product may be damaged, or residues may remain on the product. With respect to this, the patent describes that it is preferable to attach the external base film to the flexible portion so that the attachment is not very strong. However, this is not a fundamental solution.
An adhesive material disclosed in Japanese Patent Laid-Open Publication No. 2003-231875 may be employed to produce an adhesive tape used in the present invention.